6 POWER DISTRIBUTION FOR ELECTRIC RAILROADS. 
It is often convenient to have some simple expression 
conuecting the area of a wire with its weight, so that the 
latter may be readily taken into account. By a fortunate 
chance, a copper wire, 1000 ¢. 7. in section weighs almost 
exactly three pounds per 1000 ft. So if, in equation (1), 
we multiply the constant by three, and reckon I, in thous- 
ands of feet, we obtain directly the weight of conductor per 
1000 ft." Putting Ly for the length, to distinguish it from 
the former I, reckoned in feet, we have 
T DY £ L 
(4) Wm— R 
Thus, if we wish to transmit 100 amperes through 7000 
ft. of conductor at a loss of 50 volts, the conductor must 
weigh M:mz 1bs. per 1,000 ft. The total weight 
50 
of conductor is evidently Wi, Lum, and since a simple way 
of getting the total weight, without reference to wire 
tables, is often desirable, we may re-write (4), as follows: 
(5) W= 3_3__%_?31 
E 
which gives the total weight directly. ‘These weight form- 
ulee are very easy to remember and apply, and are accurate 
to about one per cent. 
The diagrams of Plate I. put equations (1), (3), (4) 
in graphic form for ready reference. Four different 
values of F, are assumed, and the unit of power is taken 
as 100 amperes. ‘The chart is therefore independent of 
the initial pressure, and serves for transmission at any 
ordinary voltage. Distances on the horizontal axis repre- 
sent length of circuit, Z. e., half the total length of con- 
ductor. ‘To find area or weight per 1000 ft. of conductor 
required for a certain distance, take an ordinate at the re- 
quired point on the distance scale and follow it up until 
it intersects the oblique line representing the assumed loss 
of voltage. ‘The area of the necessary wire can then be 
read off on the left hand scale, and the weight per 1000 ft. 
on the right. The corresponding sizes of the B. & S. 
gauge wires are annexed to the former scale. In asimilar